Diffraction element

ABSTRACT

A diffraction element having concave/convex-like diffraction gratings in its two surfaces from which at least two separated light beams can be taken in the same direction without changing largely the propagating direction of diffracted light even if the temperature of the operating environment changes. A diffraction grating having a concave/convex shape in cross-section is formed in the incoming-side surface of the transparent substrate and two diffraction gratings of concave/convex shape in cross-section are formed in the outgoing-side surface wherein the grating pitch of the first one is made equal to the grating pitch of one of the second ones. 
     In addition, a reflection type diffraction element exhibiting a good wavelength dependence of diffraction efficiency without being dependent largely on the direction of polarization of an incoming light is provided. A pseudo sawtooth-like diffraction grating is formed in either surface of the transparent substrate, a reflective film is formed on a diffraction grating portion, and an antireflective film is formed on the opposite surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/798,556, filed Mar. 12, 2004, which is a continuation ofPCT/JP02/09370, filed Sep. 12, 2002, based upon and claims the benefitof priority from the prior Japanese Patent Application Nos. 2001-278063,filed Sep. 13, 2001 and 2002-112162, filed Apr. 15, 2002. Each of theseapplications is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a diffraction element, particularly, adiffraction element having a concave/convex-like diffraction grating inits both surfaces usable for an apparatus for measuring thecharacteristics of an incoming light by separating the incoming light tothe diffraction element, and a reflection type diffraction element usedfor an apparatus for optical multiple communication, spectroscopicmeasurement and so on.

2. Background Art

Description will first be made as to a diffraction element having aconcave/convex-like diffraction grating in its two surfaces. As methodsfor measuring the characteristics of an incoming light by separating apart of the incoming light to the diffraction element into a differentdirection, there has been known a system for measuring light byseparating it by means of a prism. FIG. 6 is an illustration ofseparating light by means of a corner-cube prism wherein a prism 601comprises two corner-cube prisms; a multilayer film 602 designed so asto obtain a predetermined amount of separated light is coated on theinclined plane of either prism, and the two prisms are jointed by aresinous bonding material 603. When an incoming light 604 is incidentinto inclined planes of the two prisms at 45°, a part of the incominglight is separated as a first reflected light 605 perpendicular to theincoming light 604, by the multilayer film 602. In order to furtherseparate the first reflected light 605, it is necessary to provide aprism on the light path. For example, a prism 607 having a multilayerfilm 606 designed to obtain an appropriate amount of separated light isprovided on the light path of the first reflected light 605. By thisarrangement, the incoming light can be separated into three groups oflight beams: the incoming light 604, the first reflected light 605 andthe second reflected light 608. Each of the separated first reflectedlight 605 and the second reflected light 608 is introduced into each ofdifferent measuring apparatuses 609, 610 so that characteristics such asintensities of the incoming light can be measured independently.

Next, a reflection type diffraction element will be described. There isa method for measuring the intensity of an incoming light containingvarious wavelengths by separating and diffracting the light containingvarious wavelengths into different directions depending on thewavelengths. As the method for separating the light based on itswavelengths, there has been known a method using a reflection typediffraction element having a sawtooth-like grating in a cross-sectionalshape.

FIG. 10 shows an example of the structure of a conventional reflectiontype diffraction element using a resin. This reflection type diffractionelement 80 is formed by pressing a metal mold, in which an offsetsawtooth-like diffraction grating comprising 250 to 1,600 linearsawtooth-like grating portions per mm is precisely formed, to a resinformed on a surface of a glass substrate 801 to transfer the surfaceconfiguration of the metal mold to the resin to thereby prepare asawtooth-like diffraction grating 802, and then, by coating ahigh-reflective layer 803 thereon. The diffraction element is adapted toreceive light from the side of the sawtooth-like diffraction grating802.

As a diffraction grating having the same function, there is a pseudosawtooth-like diffraction grating in which the sawtooth-like shape isapproximated by stairs. This device is prepared by using techniques ofphotolithography and dry etching. FIG. 11 shows a reflection typediffraction element prepared by these techniques. In the same manner asshown in FIG. 10, a pseudo sawtooth-like diffraction grating 902 isformed on a glass substrate 901 and a reflective layer 903 is coatedthereon whereby a reflection type sawtooth-like diffraction element 90is provided. In FIGS. 10 and 11, arrow marks of solid line indicate anincoming light, arrow marks of one-dotted chain line indicate areflected light and arrow marks of broken line indicate a 1st orderdiffraction light, respectively. The diffraction element used here isadapted to receive light from the side of the reflection typediffraction element 90.

As materials for these diffraction gratings, a glass substrate, aninorganic film or the like can be used other than the resin as used inthe embodiment shown in FIG. 10.

Since these elements are reflection type diffraction elements, anincoming light is separated so as to be reflectively diffracted.Accordingly, from structural restriction in many cases, light isincident, from an upper side of the diffraction elements, into thepseudo sawtooth-like or the sawtooth-like diffraction grating at anincident angle θ of from 30° to 45° with respect to the normal line seton each grating surface. FIGS. 12 and 13 show examples of the wavelengthdependence of the diffraction efficiency of the reflection typediffraction elements each comprising the sawtooth-like diffractiongrating using a resin or the pseudo sawtooth-like diffraction gratingusing an inorganic film when light is incident at θ=40°. Here, outlinedcircles denote an S-polarized light and black circles denote aP-polarized light in both Figures.

Description will first be made as to the diffraction element havingconcave/convex-like diffraction gratings in its two surfaces. Suchelement can separate an incoming light into two or more portions byusing a plurality of corner-cube prisms as shown in FIG. 6. However, anadditional prism is needed as the number of times of separation of theincoming light increases. Therefore, there was a problem that it wasdifficult to constitute the separation system having a reduced size andbeing excellent in mass production. Further, there was also a problemthat it was difficult to lead two or more separated portions of lightinto the same direction because the corner-cube prism is adaptedbasically to separate light into an orthogonal direction (the reflectedlight is orthogonal with respect to the incoming light). In addition,since a resinous adhesive material was used for the prism, there was aproblem of causing the deterioration of transmission/reflectioncharacteristics due to the deterioration of the adhesive material or thecontamination of the optical plane around the adhesive material due toevaporation of a component in the adhesive material if the prism wasused for a long period or it was located in a poor environment.

Next, description will be made as to the reflection type diffractionelement. When a resin having an excellent transferability is used as thematerial for the diffraction grating, there is obtainable a reflectiontype diffraction element in which the diffraction efficiency is notchanged largely depending on the wavelength of light and is notinfluenced substantially by the polarization direction, as shown in FIG.12. However, the use of such resin created a problem that the elementwas deteriorated in a condition of high temperature or hightemperature/high humidity, hence durability was insufficient and it wasusable only in limited circumstances. Further, since the element wasproduced by a precise transfer process, there was problems thatproductivity was low and an element of high performance could not beproduced on a low price and a large scale.

On the other hand, in the reflection type diffraction element formed byprocessing the substrate composed of an inorganic material or aninorganic material formed on the substrate into a stair-like shape, theelement which is reliable, excellent in productivity and inexpensive canbe produced on a large scale production. However, in such reflectiontype diffraction element, the wavelength dependence of the diffractionefficiency depends largely on polarization directions as shown in FIG.13. Accordingly, there is a large fluctuation in spectroscopic signalsin practical use. Further, there are problems that it is necessary toreduce the incident angle in order to assure good polarizationdependence and wavelength dependence with respect to an incoming light,and a large angle can not be provided between an incoming light and adiffracted light. Accordingly, there was restriction in designing thearrangement of the spectroscopic system.

In either element using a resinous film or an inorganic film, there wassuch a problem that when a highly reflective layer formed on a substratesurface was damaged or stained, the optical characteristics would bedeteriorated remarkably. In addition, there was such a problem that whenthe high-reflective layer providing a sufficient reflecting property wasformed, the original shape of the grating could not be kept depending ona state of adhesion of the layer and the characteristics according todesigned values with respect to the grating could not be satisfied.

It is an object of the present invention to solve the above-mentionedproblems and to provide a diffraction element, in particular, adiffraction element having concave/convex-like diffraction gratings inits two surfaces, which can be a small-sized light separating deviceexcellent in mass production, wherein two separated portions of lightcan easily be taken in the same direction and influences little on theoptical system by the adhesive around it.

Further, it is an object of the present invention to provide areflection type diffraction element in which there is a low possibilityof causing a change of the diffraction efficiency depending on thepolarization direction and the wavelength of an incoming light and whichis excellent in mass production and reliability.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, there is provided adiffraction element comprising a diffraction grating having aconcave/convex shape in cross-section formed in a surface or bothsurfaces of a transparent substrate, wherein the diffraction element isadapted to receive light through the surface of the transparentsubstrate, which is opposite to the surface in which the diffractiongrating is formed, in a case that the diffraction grating is formed ineither one surface, and is adapted to receive light through the surface,in which the diffraction grating is formed in its central region, of thetransparent substrate, in a case that diffraction gratings are formed inthe both surfaces wherein a diffraction grating is formed in the centralregion of at least one of the surfaces.

Further, there is provided the above-mentioned diffraction elementcomprising the transparent substrate and the diffraction gratings havinga concave/convex shape in cross-section formed in the both surfaces ofthe transparent substrate, wherein an incoming-side surface, into whichan external light is incident, in the both surfaces of the transparentsubstrate is provided with an incoming-side diffraction grating in itscentral region, and at least one outgoing-side diffraction grating isformed in an outgoing-side surface which is opposite to saidincoming-side surface, and at least one of outgoing-side diffractiongratings is formed on the light path of the external light diffracted bysaid incoming-side diffraction grating, and the grating pitch thereof issubstantially equal to the grating pitch of the incoming-sidediffraction grating.

Further, there is provided the above-mentioned diffraction elementwherein the diffraction grating is formed directly in the surface of thetransparent substrate.

Further, there is provided the above-mentioned diffraction elementwherein the diffraction grating is formed in an inorganic film formed ona surface of the transparent substrate.

Further, there is provided the above-mentioned diffraction elementwherein at least one of the outgoing-side diffraction gratings, whosegrating pitch is substantially equal to the grating pitch of theincoming-side diffraction grating, is a reflection type diffractiongrating.

Further, there is provided the above-mentioned diffraction elementwherein at least one of the outgoing-side diffraction gratings, whosegrating pitch is substantially equal to the grating pitch of theincoming-side diffraction grating, is a diffraction grating having asaw-tooth like concave/convex portion or a pseudo sawtooth-likediffraction grating wherein a saw-tooth like shape is approximated bystairs.

Further, there is provided the above-mentioned diffraction elementwherein in the pseudo sawtooth-like diffraction grating, the height orthe depth of a step is different from the height or the depth of anotherstep, these steps constituting the stairs.

Further, according to the present invention, there is provided areflection type diffraction element comprising the above-mentioneddiffraction grating wherein a reflective film is formed on theconcave/convex portion in cross-section of the diffraction gratingformed in one surface of the transparent substrate and an antireflectivefilm is formed on the surface of the transparent substrate, which isopposite to the surface in which the concave/convex portion is formed,whereby it is adapted to receive light from the side of theantireflective film.

Further, there is provided the above-mentioned reflection typediffraction element wherein a protecting member composed of an inorganicmaterial or an organic material is provided on the transparent substrateat the side of the reflective film so as to protect the reflective film.

Further, there is provided the above-mentioned reflection typediffraction element wherein the transparent substrate is a glasssubstrate, and the concave/convex portion is formed directly in asurface of the glass substrate or is formed in the inorganic materialformed on the surface of the glass substrate.

Further, there is provided the above-mentioned reflection typediffraction element wherein the cross-sectioned shape of theconcave/convex portion is a sawtooth-like shape or a shape in which asawtooth-like shape is approximated by stairs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the structure of thediffraction element according to Example 1.

FIG. 2 is a cross-sectional view showing the structure of thediffraction element according to Example 2.

FIG. 3 is a schematic cross-sectional view showing an embodiment of thestructure of the diffraction element according to the present invention.

FIG. 4 is a schematic cross-sectional view showing another embodiment ofthe structure of the diffraction element according to the presentinvention.

FIG. 5 are graphs showing examples of the diffraction characteristics ofa differently divided grating pitch of the diffraction element of thepresent invention wherein (a) shows a case that the grating pitch isdivided to be equal and (b) is a case that the divided portion of thegrating pitch is modified.

FIG. 6 is a schematic view showing an embodiment of separating light byusing a conventional corner-cube prism.

FIG. 7 is a side view showing the structure of the reflection typediffraction element according to Example 3.

FIG. 8 is a side view showing an embodiment of the structure of thereflection type diffraction element according to the present invention.

FIG. 9 is a graph showing an example of the diffraction characteristicsof the reflection type diffraction element according to the presentinvention.

FIG. 10 is a side view showing the structure of a conventionalreflection type diffraction element.

FIG. 11 is a side view showing the structure of another conventionalreflection type diffraction element.

FIG. 12 is a graph showing an example of the diffraction characteristicsof a conventional reflection type diffraction element.

FIG. 13 is a graph showing another example of the diffractioncharacteristics of a conventional reflection type diffraction element.

BEST MODE FOR CARRYING OUT THE INVENTION

The diffraction element having concave/convex-like diffraction gratingsin its two surfaces according to the present invention will first beexplained. The present invention relates to a diffraction element havingdiffraction gratings in both surfaces of a transparent substrate whereina surface of the transparent substrate is processed in aconcave/convex-like shape in cross-section but the plane of it is alinear form or a curved form, and accordingly, the diffraction gratinghas a concave/convex portion. Further, the diffraction grating having aconcave/convex portion may be formed by processing an inorganic filmformed on a surface of the transparent substrate.

The diffraction element of the present invention is provided with anincoming-side diffraction grating in the central region of anincoming-side surface, into which an external light is incident, in thetwo surfaces, and at least one outgoing-side diffraction grating formedin an outgoing-side surface which is opposed to the incoming-sidesurface. These gratings are the diffraction gratings each having theconcave/convex portion as described above.

Further, in the diffraction element of the present invention, the atleast one outgoing-side diffraction grating is formed on the opticalpath of the external light diffracted by the incoming-side diffractiongrating, and the grating pitch thereof is substantially equal to thegrating pitch of the incoming-side diffraction grating.

Namely, the position of the outgoing-side diffraction grating isdetermined to be a position in the outgoing-side surface, at which theexternal light diffracted by the incoming-side diffraction gratingreaches through the transparent substrate. Further depending on apurpose, two outgoing-side diffraction gratings may be provided on theoptical paths along which diffracted external lights propagate. In thiscase, the grating pitch of the incoming-side diffraction grating issubstantially equal to at least one of the two outgoing-side diffractiongratings. Here, “substantially equal” means that a difference betweengrating pitches of the incoming-side diffraction grating and theoutgoing-side diffraction grating is 0.5° or less in terms of adifference of diffraction angle.

By constructing it as described above, the diffraction element of thepresent invention provides such effect that a change in a propagationdirection of the diffracted light when there is a variation of thewavelength, is small.

Further, in the diffraction element of the present invention, when theat least one outgoing-side diffraction grating having the substantiallyequal grating pitch is a reflection type diffraction grating, aphotodetector for detecting the diffracted light by the reflection typediffraction grating can be placed at the side of receiving an externallight. With such arrangement, the system including an external lightsource, the diffraction gratings, the photodetector and so on canpreferably be miniaturized.

In the following, an embodiment of the diffraction element of thepresent invention will be described with reference to the drawing. FIG.3 shows an embodiment of the structure of the diffraction element of thepresent invention. An incoming-side diffraction grating 302 havingrectangular concave/convex portions is formed by, for example, aphotolithography method and a dry etching method in the central regionof an incoming-side surface, into which an external light is incident,of a glass substrate as a transparent substrate 301. An incoming light303 incident perpendicularly into the incoming-side surface is separatedinto three light beams: the incoming light 303 passing straight throughthe glass substrate, and a +1st order diffraction light 304 and a −1storder diffraction light 305 which are generated by the incoming-sidediffraction grating 302.

The light quantity of each diffraction light can be distributed so thateach diffraction light quantity is lessened or the almost quantity canbe the diffraction light by adjusting the depth of the incoming-sidediffraction grating 302 by processing. The incoming-side diffractiongrating 302 may be formed in substantially the entire region excluding aperipheral region where the intensity of the light flux of the incominglight 303 is weak or it may be formed only a part of such region. Theintensity of the actual diffraction light is determined based on thediffraction efficiency of the diffraction grating and the surface areaof the diffraction grating with respect to the cross-sectional area ofthe light flux.

The incoming light 303 passing through the diffraction element, after ithas transmitted to the outgoing-side surface of the transparentsubstrate 301, is used as, for example, light beams forrecording/reproducing information of an optical disk or light beams foroptical communication. On the other hand, the +1st order diffractionlight 304 and the −1st order diffraction light 305 generated by theincoming-side diffraction grating 302 propagate obliquely at an angleindicated by formula 1 in the transparent substrate 301 to reach theoutgoing-side surface of the glass substrate. In formula 1, θ3represents an angle of the propagating light in the transparentsubstrate, λ represents the wavelength of the incoming light, P1represents the grating pitch of the incoming-side diffraction grating, nrepresents the refractive index of the transparent substrate in a caseof λ, and m represents an order of diffraction.

On optical paths of the +1st order diffraction light 304 and the −1storder diffraction light 305 at the outgoing-side surface, a firstoutgoing-side diffraction grating 306 for the diffraction light 304 anda second outgoing-side diffraction grating 307 for the diffraction light305 are formed respectively. Here, the grating pitch of theincoming-side diffraction grating 302 is equal to grating pitches of thefirst and second outgoing-side diffraction gratings 306, 307. Thediffraction light 304 and the diffraction 305 are respectivelydiffracted to propagate into directions determined by formula 2 by meansof the first and second outgoing-side diffraction gratings 306, 307respectively. In formula 2, θ4 represents an angle of the propagatinglight in the transparent substrate, i represents an incident angle inthe transparent substrate, λ represents the wavelength of the incominglight, P2 represents the grating pitch of the outgoing-side diffractiongrating, n represents the refractive index of the transparent substratein a case of λ, and m represents an order of diffraction.

sin(θ3)=m×λ/(P1·n)  Formula 1

sin(θ4)−sin(i)=m×λ/(P2·n)  Formula 2

As shown in FIG. 3, either of two outgoing-side diffraction gratings,for example, the outgoing-side diffraction grating 306 may be atransmission type diffraction grating and the other, the outgoing-sidediffraction grating 307, may be a reflection type diffraction gratingformed by coating a reflective film 308. On the contrary, theoutgoing-side diffraction grating 306 may be a reflection typediffraction grating.

Each light diffracted by the outgoing-side diffraction grating 306 or307 is introduced into each photodetector or the like. When thewavelength of the incoming light 303 varies, angles θ3, θ4 ofpropagating light are changed, as shown in Formulae 1 and 2, so thatdirections of propagation of the diffraction lights are changed. In casethat a measuring device such as a photodetector having an incident angledependence is used or that the distance between a measuring device andthe diffraction element is large even though the photodetector does nothave an incident angle dependence, measurement error may be caused dueto wavelength variation, or the position of the incoming light to thedetecting portion of the measuring device may change, depending on thewavelength dependence of the propagation direction (angle).

When the grating pitch of the incoming-side diffraction grating is madeequal to the grating pitch of the outgoing-side diffraction grating, the−1st order diffraction light 309 generated by the outgoing-sidediffraction grating 306 is used for the diffraction light 304 generatedby the incoming-side diffraction grating 302, and the +1st orderdiffraction light 310 generated by the outgoing-side diffraction grating307 is used for the diffraction light 305 generated by the incoming-sidediffraction grating 302, whereby changes in propagation direction causedby a change of the wavelength of the incoming light can be canceled.

Depending on measuring devices used, the grating pitch of both theoutgoing-side diffraction grating 306 and outgoing-side diffractiongrating 307 may be in conformity with the grating pitch of theincoming-side diffraction grating 302 or the grating pitch of eithernecessary one may be in conformity with it. When the grating pitches aremade equal to cancel the wavelength dependence in propagation directionof incoming light beams, the −1st order diffraction light 309 generatedby the outgoing-side diffraction grating 306 and the +1st orderdiffraction light 310 by the outgoing-side diffraction grating 307 arenecessarily used, and a diffraction light having an order other than theabove-mentioned can not be used because it has wavelength dependence inthe propagation direction.

In this case, it is preferable to use, as the outgoing-side diffractiongrating having substantially equal grating pitch, a sawtooth-likediffraction grating or a pseudo sawtooth-like diffraction grating whoseshape is approximated by stairs, each of which exhibits a highdiffraction efficiency with respect to a diffraction light having aspecified order of diffraction, because utilization efficiency of lightfor a system for recording/reproducing light or optical communicationcan be increased. As the pseudo sawtooth-like shape, a continuousinclined plane of a sawtooth can be approximated by a plurality of stepslike stairs. This pseudo sawtooth-like diffraction grating matches wellmanufacturing processes such as a photolithography method and a dryetching method. Here, the number of steps is determined depending onconditions of using the diffraction element. Usually, two steps to 31steps are used.

In addition to the structure of the above-mentioned diffractiongratings, such structure may be adopted wherein the outgoing-sidediffraction gratings 306, 307 are formed in the incoming-side surfaceand a reflective film is formed at portions corresponding to gratingplanes of the outgoing-side diffraction gratings into which thediffraction light 304 and the diffraction light 305 generated by theincoming-side diffraction grating 302 incident so that the light beamsare diffractively returned.

The sawtooth-like or the pseudo sawtooth-like diffraction grating can beused for the incoming-side diffraction grating 302. In this case, it ispossible to assign the intensity of each diffraction light diffracted toeach of the two measuring devices. Accordingly, it is possible to makethe light beams incident into a measuring device requiring a largerlight intensity with a larger distribution ratio without reducinglargely the whole utilization efficiency of light.

In order to diffract light of a specified order of diffraction at thepseudo sawtooth-like diffraction grating, it has been known that it isthe best to divide the depth of the grating and the length in theperiodical direction of the grating to be an equal division so that ahigh diffraction efficiency can be obtained. On the other hand, in orderto increase the intensity of a 0 order diffraction light (a transmissionlight) to the maximum and to distribute a part of the incoming light bydiffraction, it is not always necessary to divide the depth of thegrating and the length in the periodical direction of the grating to bean equal division. Accordingly, the division of the depth of the gratingand the length in the periodical direction of the grating can freely bedetermined in order to obtain a desired distribution ratio on the lightquantity of diffraction light having a required order of diffraction.

For example, when the diffraction efficiency of the incoming-sidediffraction grating 302 in FIG. 3 is determined to be low so that themost part of the incoming light to the element is utilized by passing ittherethrough, a larger distribution ratio than that of the equallydivided pseudo sawtooth-like diffraction grating is obtainable byadjusting the division of the incoming-side diffraction grating 302.

FIG. 5 shows the relation between the diffraction efficiency and thetransmittance of each of the +1st order diffraction light and the −1storder diffraction light with respect to the polarization in parallel toa longitudinal direction of the grating under conditions of a wavelengthof 1550 nm and a grating pitch of 1.6 μm, as an example. FIG. 5( a)shows a case that the division of the depth of the grating and thelength in the periodical direction of the grating is made equal, andFIG. 5( b) shows a case that the division of the length in theperiodical direction of the grating is adjusted to be 1:3:1.

From the graphs of FIGS. 5( a) and 5(b), it is understood that thediffraction efficiency of the −1st order diffraction light indicated bya solid line can be increased to be higher than the diffractionefficiency of the +1st order diffraction light indicated by a brokenline with respect to the same value of transmittance. Accordingly, it ispossible to distribute a required light quantity to a measuring devicerequiring a necessary light quantity without reducing the transmittance.Thus, by adjusting the division of the depth of the grating and thelength in the periodical direction of the grating, diffractionefficiencies of the +1st order diffraction light and the −1st orderdiffraction light can be changed together. The degree of change isgreater as the grating pitch approaches the wavelength.

FIG. 4 is a cross-sectional view showing another embodiment of thestructure of the diffraction element of the present invention. In orderto control the direction of diffraction light beams separated by thediffraction, the light beams may be introduced obliquely in theabove-mentioned embodiment shown in FIG. 3. The other embodiment shownin FIG. 4 has such a structure that two diffraction lights diffractedand separated by the incoming-side diffraction grating are returnedtoward the incoming-side by being diffracted by outgoing-sidediffraction gratings. An incoming light 403 incident obliquely to anincoming-side diffraction grating 402 formed in an incoming-side surfaceof a transparent substrate 401 is diffracted to be a +1 st orderdiffraction light 404 and a −1st order diffraction light 405. Thegenerated diffraction light 404 and diffraction light 405 are diffractedreflectively at outgoing-side diffraction gratings 406, 407 withreflective films 408, which are formed on an outgoing-side surface, tobe emitted as returning lights 409, 410 through the incoming-sidesurface.

In a case that the incoming light 403 is inclined toward the −1st orderdiffraction light 405 as shown in FIG. 4, and the grating pitch of theincoming-side diffraction grating 402 is equal to the grating pitch ofthe outgoing-side diffraction grating 406, the direction of thereturning light 409 has a reverse inclination with respect to thedirection of the incoming light, and the angle of the returning light tothe normal line extending from the diffraction element is twice as largeas the incident angle. Accordingly, the returning light 409 does notcross the incoming light 403 and it is separated at an angle three timeslarger than the incident angle. This angular relation is maintained evenif the wavelength of the incoming light varies.

On the other hand, when the grating pitch of the outgoing-sidediffraction grating 407 to which the diffraction light 405 is incident,is determined appropriately to be wider than the grating pitch of theincoming-side diffraction grating 402, the returning light 410 can beemitted from the element in substantially parallel without crossing theincoming light 403.

By using this method, the propagation direction of light beams canfreely be determined even when a measuring device having an incidentangle dependence is used. By changing the grating pitch of thediffraction grating, the direction of diffraction can be changed.However, a high diffraction efficiency can not be obtained in a regionthat the grating pitch is closer to the wavelength, and actual work forpreparing the grating becomes difficult. Accordingly, the way ofcontrolling the propagation direction of light beams by introducinglight with an oblique angle becomes effective as described above.

Namely, in the diffraction element, it is preferred to determine thegrating pitch of the outgoing-side diffraction grating or theincoming-side diffraction grating so that the angle between thedirection of an incoming light and the direction of either one of the+1st order diffraction light or the −1st order diffraction lightgenerated by the incoming-side diffraction grating, becomes larger in adirection from the incoming-side surface toward the propagationdirection when an external light is introduced obliquely to the surfaceof the incoming-side diffraction grating formed in the incoming-sidesurface of the diffraction element.

The sawtooth-like or pseudo sawtooth-like diffraction grating can beused as the incoming-side diffraction grating 402. In such case, thedistribution ratio of the light quantity to each of the two measuringdevices can be changed, and light can be introduced to a measuringdevice requiring a larger intensity with a larger distribution ratio ofthe intensity without reducing largely the whole utilization efficiencyof light. In the same manner as the first embodiment, when thediffraction efficiency of the incoming-side diffraction grating 402 isdetermined to be low to utilize the most part of the incoming light tothe element by passing it therethrough, it is possible to make thedistribution ratio larger than that of the equally divided pseudosawtooth-like diffraction grating by adjusting the division of theincoming-side diffraction grating 402.

By using the structure of the present invention, light beams can beseparated and propagated with a higher degree of freedom by asmall-sized diffraction element excellent in mass production andreliability, and the reduction of the wavelength dependence in apropagation direction and the degree of freedom of directions ofseparating light beams can be satisfied at the same time as the caserequires. A spectroscopic system excellent in reliability and massproduction can be realized by using the diffraction element having thewavelength dependence in the diffraction direction in principle.

The grating pattern of the diffraction element of the present inventionis prepared by using, for example, a photomask. Accordingly, not only alinear shape but also a shape along a curved line can be formed. Thus,in designing a radius of curvature for the grating pattern, it ispossible to add function as a lens so that a diffraction light can becollected on a photodetector. Further, by using a process for a waferhaving a large surface area, a layer having function as a phase platecan be laminated whereby a diffraction element of highperformance/composite type can be produced.

The diffraction grating formed in the diffraction element of the presentinvention may be formed in the transparent substrate itself and/or afilm formed on the transparent substrate. It is preferable fromviewpoints of reliability and a large scale production to processdirectly the transparent substrate excellent in being etched becausecost for forming the film does not occur and there is no needlessinterface. As material for the transparent substrate, quartz glassexhibiting a high transparency in a broader wavelength region coveringultraviolet light, visible light and infrared light can be mentioned asan example. However, when the usable wavelength is only in an infraredregion, a silicone substrate or the like exhibiting a high transparencyin the infrared region although it is not transparent in a visible lightregion can be used. In order to control further the change of thedirection of separation/propagation in response to a change of operatingenvironmental temperature, a diffraction grating formed in a transparentsubstrate having a low expansion coefficient by processing thetransparent substrate directly or a film formed on the transparentsubstrate, is preferably used.

When a diffraction grating formed in the diffraction element is used asa reflection type diffraction grating, a reflective interface is formedat the diffraction grating. In this case, a dielectric multilayer filmor a metal film composed of a reflective interface material may beformed, in particular, the metal film is more preferably used because itprovides a high reflection efficiency with a thinner film thickness. Inorder to form a film of fine structure, it is preferable to use asputtering method or a RF vacuum deposition method providing anexcellent wrapping performance to the reflective interface material atthe time of the formation of a film. Further, a wet process such as aplating method may be used.

The present invention provides a better effect in using a narrow-pitcheddiffraction grating by which an amount of separation of a diffractionlight is increased by increasing the diffraction angle. In particular, alarger effect is obtainable by a diffraction grating in which thegrating pitch is about two times or less of the center-wavelength.

Next, description will be made as to the reflection type diffractionelement of the present invention wherein a light reflective film isformed on a concave/convex portion of the diffraction grating having aconcave/convex-like shape in cross-section which is formed in a surfaceof a transparent substrate. Further, an antireflective film is formed onthe opposite surface of the transparent substrate, in which theconcave/convex portion is formed whereby the reflection type diffractionelement is adapted to receive light incident from the side of theantireflective film.

By constructing the reflection type diffraction element, the change ofthe diffraction efficiency due to the wavelength dependence can besuppressed, and an irregular polarization direction can also besuppressed. The concave/convex portion may have a rectangular shape, asawtooth-like shape or a pseudo sawtooth-like shape. Although the effectexpected for the present invention can be achieved by using any type ofthese shapes, use of the sawtooth-like shape or a pseudo sawtooth-likeshape can provide a higher diffraction efficiency of a diffraction lightin a wider range of the wavelength and the angle of the incoming lightor a wider range of the grating pitch.

In the following, description will be made by taking the sawtooth-likeshape or the pseudo sawtooth-like shape as an example with reference tothe drawing.

FIG. 8 is a side view showing an embodiment of the structure of thereflection type diffraction element according to the present invention.A low-reflective film 703 as an antireflective film is coated on atransparent substrate 701 so that this film constitutes a lightreceiving plane. In a rear surface of the transparent substrate 701without having any low-reflective film 703, a pseudo sawtooth-likediffraction grating 702 having a grating pitch P whose sawtooth-likeconcave/convex portion is approximated by a stair-like shape of fourlevels (three steps), is formed by repeating photographic and dryetching processes. On this pseudo sawtooth-like diffraction grating, ahigh-reflective layer 704 as a reflective film made of metal is formed.

Further, a protecting substrate 706 is bonded onto the high-reflectivelayer 704 by means of an adhesive layer 705 to protect thehigh-reflective layer 704. Thus, a reflection type diffraction element70 is constituted. When light having a wavelength λ is incident into thelow-reflective film 703 of the reflection type diffraction element 70with an external incident angle θ1 with respect to the normal line, thelight is diffracted by the transparent substrate 701 having a refractiveindex n and propagates in the transparent substrate 701 with an internalincident angle θ2 according to Snell's law, sin(θ2)=sin(θ1)/n.

The propagating light is incident into the pseudo sawtooth-likediffraction grating 702 with an internal incident angle θ2 whereby thealmost amount of the propagating light is concentrated and diffractedreflectively as diffraction light having a sign, i.e., diffraction lighthaving a +sign or a −sign with a specified diffraction order, which isdetermined by the shape of the grating. In a case that the diffractionlight is concentrated to the light having a −1st order, it propagates inthe transparent substrate 701 with a diffraction angle Φ2 with respectto the normal line, determined by the following formulae, and isdiffracted at the interface between the low-reflective film 703 and airso as to propagate in the air with a diffraction angle Φ1 toward adetector (not shown). Here, a solid arrow mark indicates an incominglight, a one-dotted chain line indicates the reflected light and abroken line indicates a 1st order diffraction light, respectively.

sin(Φ2)−sin(θ2)=λ/P  Formula 3

sin(Φ1)=sin(Φ2)×n  Formula 4

Here, Φ1 is the same as the diffraction direction of the reflection typediffraction element having a diffraction grating on a surface of thetransparent substrate in consequence.

FIG. 9 shows wavelength dependences on the diffraction efficiencydepending on directions of different polarization under conditions thata pseudo sawtooth-like diffraction grating comprising about 600 gratingsper mm is formed in a rear plane (a plane without having thelow-reflective film) of a quartz glass substrate having a refractiveindex of 1.44 and an external incident angle θ is 40°. Here, outlinedcircles indicate an S-polarized light and black circles indicate aP-polarized light. It is understood that the wavelength dependences onthe diffraction efficiency depending on the difference of directions ofpolarization can be improved in FIG. 9 in comparison with the reflectiontype diffraction element having the pseudo sawtooth-like diffractiongrating under the condition of the same incident angle, as shown in FIG.13.

In the structure of the present invention, even when light is introducedinto the reflection type diffraction element with a larger externalincident angle, it can be directed to the pseudo sawtooth-likediffraction grating with a smaller internal incident angle in thetransparent substrate, with the result that wavelength dependences onthe directions of polarization can be reduced. With this structure, thereflection type diffraction element excellent in reliability and massproduction can be realized, and a more inexpensive spectroscopic systemcan be realized. Since it is particularly unnecessary to determine asmaller external incident angle in the reflection type diffractionelement in order to assure the diffraction efficiency without relyinglargely on the direction of polarization and the change of thewavelength of an incident light, a larger degree of freedom can bepresented for designing the spectroscopic system.

The pattern of diffraction grating, in a plane view, of the reflectiontype diffraction element of the present invention can be prepared byusing a photomask or the like. Accordingly, such diffraction gratingpattern is not limited to a linear shape but can be, for example, acurved shape. Such curved shape allows to add function as a lens so thatthe diffraction light can be collected onto a detector. Further, byusing a process for a wafer having large surface area, it is possible tolaminate an optical layer having another function such as a phase plateon the reflection type diffraction element. In this case, it can be of ahigh performance and a complex system.

The diffraction grating of the present invention is preferably preparedby processing a glass substrate itself or an inorganic material formedon the glass substrate. It is particularly preferable to processdirectly a quartz glass substrate having high speed and uniform etchingcharacteristics in the points that cost for film formation does notoccur and the interface between the film and the substrate does notexist. Further, it is also preferable from viewpoints of reliability anda large scale production. Further, when an inorganic material having anexcellent etching property is formed as a film on a quartz glasssubstrate having a smaller thermal expansion coefficient than theorganic material, there is such effect that a change of the diffractiondirection due to a temperature change can be controlled. Such structureis preferred to obtain the element having excellent temperaturecharacteristics.

In the high-reflective film, there is no limitation as to its thicknessbecause only the interface of the film to the sawtooth-like diffractiongrating functions optically. Accordingly, it is unnecessary to considerthe deterioration (deformation) of the shape of the high-reflective filmeven if it has a large thickness, and accordingly, it may be formed tohave a sufficient film thickness. In this case also, a sufficientreflectivity can be assured. Further, it is unnecessary to form it to bethin with high accuracy, use of a vacuum film-forming method such as avacuum deposition method, a sputtering method or the like is not alwaysnecessary but a plating method or the like may be used.

The protecting means for protecting the high-reflective film formed onthe rear surface of the reflection type diffraction element does notfunction optically. Accordingly, the protecting means is unnecessarilyto be transparent and there is no limitation on thickness. It ispreferable that a protecting means composed of an inorganic material oran organic material is provided at the side of the high-reflective film.Although an organic material such as a resin or an inorganic materialallowing a vacuum film-forming method can widely be employed, a resinousmaterial which can be coated and cured is particularly preferably used.

When the reflection type diffraction element is mounted on a specifiedapparatus wherein, for example, the rear surface of the reflection typediffraction element is required to have accuracy in order to use therear surface as standard at the time of mounting, or a strongerprotection is required, it is preferable to bond a flat and strongertransparent substrate by using an adhesive coated on the high-reflectivefilm.

In this case, a glass substrate, a silicone substrate or the like can beused. However, it is preferable to use a material having substantiallythe same thermal expansion coefficient as the transparent substrate inorder to suppress a change in the characteristics caused during a hightemperature condition, resulted due to difference in the expansioncoefficient. In the present invention, a remarkable effect is obtainablewhen a diffraction grating having a narrow pitch, which improves thewavelength dissolving power by increasing especially the diffractionangle, is used. In particular, use of such diffraction grating that thegrating pitch is substantially equal to the center-wavelength of lightor that the grating pitch is smaller than the center-wavelength oflight, provides a large effect.

In the following, some examples will be described.

EXAMPLE 1

FIG. 1 is a cross-sectional view showing the structure of thediffraction element of this example. In this Example, a quartz glasssubstrate having a thickness of 2.0 mm was used as a transparentsubstrate 101, and a pseudo sawtooth-like diffraction grating (atransmission type) having a grating pitch of 1.15 μm and three levels(two steps), wherein each height (depth) was 0.15 μm and 0.30 μm, wasformed in a central region having a diameter of 0.5 mm ( ) in itsincoming-side surface by repeating a photolithography method and a dryetching method. Thus, an incoming-side diffraction grating 102 wasformed.

Then, in an outgoing-side surface as the opposite side in the quartzglass substrate, a three-beam generating diffraction grating 103 fordetecting a tracking signal, having a grating pitch of 20 μm and a depthof 0.2 μm was formed as an outgoing-side diffraction grating. Further, apseudo sawtooth-like diffraction grating (a reflection type) having agrating pitch of 1.15 μm, which was equal to the grating pitch of theincoming-side diffraction grating 102, and three levels (two steps),wherein each height (depth) was 0.1 μm and 0.2 μm, by processing anouter peripheral region of the three-beam generating diffraction grating103 and by coating a reflective film 104 of gold having a film thicknessof 200 nm selectively only on the grating portion by a sputtering methodand a photolithographic method, whereby a reflective diffraction grating105 was provided as another outgoing-side diffraction grating. Finally,low-reflective coating layers (not shown) were formed on both surfacesof the quartz glass substrate to obtain a diffraction element 106.

In the following, description will be made as to an optical head devicein which this diffraction element 106 is assembled. When light having anoscillation wavelength of 660 nm emitted from a semiconductor laser 107was incident into the diffraction grating, only the center portion ofthe light which had a stronger intensity at the incoming-side surface ofthe diffraction element was passed through the incoming-side diffractiongrating 102, and the other part of light was diffracted. The centerportion of the light without being diffracted and light propagatingthrough the region other than the incoming-side diffraction grating 102and propagating rectilinearly, were respectively diffracted andseparated into three directions at the three-beam generating diffractiongrating 103 to be introduced into an optical disk by means of acollimator lens or an objective lens (not shown). On the other hand, thelight diffracted by the incoming-side diffraction grating 102 wasdirected to the reflective diffraction grating 105, and the reflectingdiffraction light emanated through the diffraction element 106 to bedetected by a receptor 108.

In this Example, about 85% of the light emitted from the semiconductorlaser 107 reached the three-beam generating diffraction grating and wasintroduced into the optical disk. On the other hand, 5% of the light wasdetected by the receptor 108 through the incoming-side diffractiongrating 102 and the reflective diffraction grating 105. In this detectedlight, no substantial change of signal level to the receptor wasobserved even in consideration of the difference of the inherency of theoscillation wavelength of the used semiconductor laser 107.

Further, the signal level was stable even in a change of the wavelengthdue to a temperature change of the semiconductor laser 107. In addition,since there was no change of the light receiving position due to achange of the incident angle derived from a wavelength variation, theadjusting unit for the receptor could be omitted. By adjusting theoscillation intensity of the semiconductor laser 107 by using the signallight to the receptor, it was possible to record information into andreproduce it from the optical disk stably.

EXAMPLE 2

FIG. 2 is a cross-sectional view showing the structure of thediffraction element of this example. In this Example, a quartz glasssubstrate having a thickness of 2.0 mm was used as a transparentsubstrate 201. By repeating a photolithography method and a dry etchingmethod to the central region having a diameter of 1.0 mmΦ of anincoming-side surface of the substrate, a pseudo sawtooth-likediffraction grating (a transmission type) having three levels (twosteps) wherein each height (depth) was 0.60 μm, 0.30 μm and 0.0 μm, andthe region of grating pitch was 1.8 μm, which was divided into threesub-regions of 0.36 μm, 1.08 μm and 0.36 μm, was formed. Thus anincoming-side diffraction grating 202 was formed.

In two regions in the opposite outgoing-side surface of the quartz glasssubstrate at which the light diffracted by the incoming-side diffractiongrating 202 reached, a reflective diffraction grating 203 as anoutgoing-side diffraction grating having a grating pitch of 1.8 μm and areflective diffraction grating 204 as another outgoing-side diffractiongrating having a grating pitch of 2.0 μm were formed by repeating aphotolithography method and a dry etching method. In the reflectivediffraction gratings 203 and 204, each of the grating pitches wasdivided into four portions and these grating pitch portions wereprocessed to form steps each having a height (depth) of 0.15 μm toprovide pseudo sawtooth-like diffraction gratings having four levels.Then, a reflective film 205 having a film thickness of 200 μm was coatedselectively only on each grating portion by sputtering gold by using alift-off method to thereby form pseudo sawtooth-like diffractiongratings (reflection type) of four levels (three steps). Finally,antireflective coating films (not shown) were coated on both surfaces ofthe quartz glass substrate. Thus a diffraction element 206 was prepared.

When a collimated incoming light 207 of a wavelength of 1550 nm having apolarization perpendicular to a longitudinal direction of thediffraction grating was incident into the diffraction element 206 at anincident angle of 5° with respect to the normal line of the diffractiongrating, the incoming light 207 was separated into three portions at theincoming-side diffraction grating 202. In this construction, about 92%of the incoming light 202 transmitted through the diffraction grating206. With respect to a +1st order diffraction light (at a left side inthe figure), about 3% of the incoming light quantity was diffracted tobe directed into the reflective diffraction grating 203 having the samegrating pitch as the incoming-side diffraction grating 202, at which thealmost amount of light was diffracted so that the diffracted lightemanated from the diffraction element 206. The angle of emanation of thelight at this moment was about double, with a reverse inclination, ofthe incident angle of the incoming light 207. The light emanated finallyfrom the diffraction grating 206 was a returning light 208 having anintensity of 2.4% of the incoming light 207.

With respect to a −1st order diffraction light (a right side of thefigure), about 1% of the incoming light quantity was diffracted to bedirected into the reflective diffraction grating 203 having a largergrating pitch than the incoming-side diffraction grating 204 at whichthe almost amount of the light was diffracted and emanated from thediffraction element 206. The direction of the emanated light at thismoment was substantially parallel to the incoming light 207. The lightemanated finally from the diffraction grating 206 was a returning light209 having an intensity of 0.7% of the incoming light 207.

The returning light 208 was introduced into a double slit typespectro-diffraction element (not shown) to measure the wavelength. Inthe measurement, the flux of the incoming light should have a highparallelism. In this case, however, the measurement of the wavelengthcould be achieved because a sufficient degree of parallelism wasmaintained even when a wavelength variation took place in the incominglight. The returning light 209 was collected onto the receptor (notshown) to measure the intensity. The measurement of the intensity couldbe achieved because the returning light was stably incident even thoughthere was a wavelength variation.

In this Example, the light was separated by using an extremely smalldiffraction element, and the almost amount of the incoming light 207could be utilized without causing a substantial reduction in theintensity while the wavelength and the intensity of the light could bemeasured correctly and simultaneously.

EXAMPLE 3

FIG. 7 is a cross-sectional view showing the structure of the reflectiontype diffraction element of this example. In this Example, a quartzglass substrate having a thickness of 0.5 mm was used as a transparentsubstrate 501, and a pseudo sawtooth-like diffraction grating was formedin a surface of the substrate by using techniques of a photolithographymethod and a dry etching method. Namely, by etching two times to obtaindepths of 0.36 μm and 0.18 μm, a pseudo sawtooth-like diffractiongrating 102 having four levels (three steps) wherein each height of onestep was 0.18 μm and the total depth (the sum of the steps) was 0.54 μm,was formed.

Then, in the surface of the transparent substrate 501 which was oppositeto the surface having the pseudo sawtooth-like diffraction grating 502,a low-reflective film 503 as an antireflective film for a wavelength of1.55 μm as the center wavelength was formed. On the pseudo sawtooth-likediffraction grating 502, a film of gold having a thickness of 0.8 μm wasformed by a vacuum vapor deposition method to provide a high-reflectivefilm 504 as a light-reflective film. On the high-reflective film 504, anepoxy adhesive was applied as an adhesive layer 505, and on the epoxyadhesive, a quartz glass substrate (a protecting member) having athickness of 0.5 mm was stacked to form a protecting substrate 506.

Then, the quartz glass substrate was rotated so that the epoxy adhesivewas made thin and uniform as the adhesive layer 505 to thereby form alaminated substrate comprising the transparent substrate 501, theprotective substrate 506 and the adhesive layer 505 interposedtherebetween. The thus prepared laminated substrate was cut into arectangular shape of 10 mm×7 mm with a dicing saw to form a reflectiontype diffraction grating 50.

Light having a wavelength of 1.55 μm was incident with an externalincident angle of θ=40° from the side of the low-reflective film 503 ofthe reflection type diffraction element 50. Diffraction efficiencies ofa polarized light in parallel to the longitudinal direction of thegrating (S polarization) and a polarized light perpendicular thereto (Ppolarization) were 73% and 70% respectively. These diffractionefficiencies are substantially equal. Even in the case of changing thewavelength of the incoming light from 1.5 μm to 1.6 μm, the change ratesof the diffraction efficiencies were small as about ±5%. In FIG. 7, asolid arrow mark indicates the incoming light, a one-dotted chain lineindicates the reflected light and a broken line indicates a 1st orderdiffraction light respectively.

INDUSTRIAL APPLICABILITY

As described above, in the diffraction element having concave/convexdiffraction gratings in both surfaces according to the presentinvention, a diffraction grating is formed in an incoming-side surfaceof a glass substrate and at least one additional diffraction grating isformed in an outgoing-side surface thereof by processing in a linearshape or a curved shape in the glass substrate or an inorganic filmformed on the glass substrate, wherein the grating pitch of thediffraction grating in the incoming-side surface is made equal to thegrating pitch of the at least one diffraction grating in theoutgoing-side surface. Accordingly, the diffraction element is of asmall size, and is excellent in mass production and durability; iscapable of taking out at least one diffracted and separated light in thesame direction, and does not change substantially the direction ofpropagation of the diffraction light even though there is a change ofoperating environmental temperature.

Further, the reflection type diffraction grating of the presentinvention exhibits excellent diffraction efficiency without dependingmuch on directions of polarization with respect to an incident angle oflight to the reflection type diffraction element in comparison with theconventional reflection type diffraction element receiving light fromthe side of the diffraction grating. In addition, the reflection typediffraction element excellent in mass production and reliability can berealized.

1. A diffraction element comprising a transparent substrate anddiffraction gratings formed in both surfaces of the transparentsubstrate and having a concave/convex shape in cross-section; wherein anincoming-side surface, into which an external light is incident, in bothsurfaces of the transparent substrate is provided with an incoming-sidediffraction grating in its central region, and two outgoing-sidediffraction gratings are formed in an outgoing-side surface which isopposite to said incoming-side surface; and wherein the twooutgoing-side diffraction gratings are formed on light paths of theexternal light diffracted by said incoming-side diffraction grating, theconcave/convex shape in cross-section of each of the two outgoing-sidediffraction gratings comprises a sawtooth-like diffraction grating or apseudo sawtooth-like diffraction grating whose shape is approximated bystairs, and a grating pitch of the incoming-side diffraction grating issubstantially equal to a grating pitch of at least one of the twooutgoing-side diffraction gratings.
 2. The diffraction element accordingto claim 1, wherein the concave/convex shape in cross-section of theincoming-side diffraction grating comprises a sawtooth-like diffractiongrating or a pseudo sawtooth-like diffraction whose shape isapproximated by stairs.
 3. The diffraction element according the claim1, wherein the diffraction gratings are formed directly in the surfacesof the transparent substrate.
 4. The diffraction element according toclaim 1, wherein the diffraction gratings are formed in an inorganicfilm formed on the surfaces of the transparent substrate.
 5. Thediffraction element according the claim 1, wherein the external light isincident obliquely with respect to a normal line of a surface of thesubstrate with the incoming-side diffraction grating formed therein; agrating pitch of one of the two outgoing-side diffraction gratings issubstantially equal to the grating pitch of the incoming-sidediffraction grating; and a pitch of the other outgoing-side diffractiongratings wider than the grating pitch of the incoming-side diffractiongrating.